First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 95

systems that employ a potentiostat whereby one can dial in a potential of interest to
examine the resulting chemistry.
In this chapter, we present a brief background account of recent progress in the
application of ab initio methods to elucidating the fundamental physicochemical pro-
cesses that take place at the electrified aqueous metal interface. Our focus is predomi-
nantly on model single-crystal surfaces at lower surface coverages.
The fundamental understanding of the electronic and chemical interactions of mol-
ecules adsorbed to and reacting on extended surfaces has been significantly enhanced
by quantum chemical studies employing metal slabs as surface models. For example,
periodic density functional theory (DFT) models of surface - adsorbate interactions
have been shown to provide considerable insight into a variety of chemical phenom-
ena, such as elucidating reaction mechanisms for a variety of heterogeneous catalytic
reactions [Honkala et al., 2005; van Santen and Neurock, 2006; Zhang and Hu, 2000]
and the vibrational behavior of adsorbates [Greeley and Mavrikakis, 2004; Loffreda
et al., 1999]. More recently, such methods have also been applied to more complex
solvated surface - adsorbate systems (by including explicit solvent molecules sur-
rounding the adsorbate and co-adsorbed on the surface) and more complex electroca-
talytic systems that include both solvent and influences from the electrochemical
potential [Cao et al., 2005; Halley et al., 1998; Janik and Neurock, 2006; Taylor
et al., 2006b; Toney et al., 1994] The virtue of quantum chemical analyses of these
more complex systems is the ability to elucidate the specific influence of how solvent
or applied potential perturbs the chemistry occurring at such surfaces from both ther-
modynamic (e.g., calculations of reaction energies) and kinetic (e.g., calculations of
activation barriers) standpoints. While we try to provide various references to work
by others in the field of theoretical electrocatalysis, many of the examples discussed
are taken from our own work.
4.3 ELECTRONIC STRUCTURE METHODS AND MODELS
FOR THE ELECTROCATALYTIC INTERFACE
4.3.1 Periodic Density Functional Theory
The results discussed here were determined using plane wave DFT calculations within
the generalized gradient approximation using the Vienna Ab initio Simulation
Program (VASP) [Kresse and Hafner, 1993; Kresse and Furthmuller, 1996a, b]. The
catalytic gas/solid interface was modeled using a standard supercell approach
where 3 - 5 metal layers are chosen to mimic the metal substrate and a vacuum
region of greater than 10 ˚ above the surface is used to represent the gas phase.
The metal atoms are arranged so as to model a specific single-crystal surface. The
unit cell was subsequently repeated in two dimensions along specific lattice vectors
parallel to the surface plane and in a third dimension perpendicular to the surface to
create parallel slabs. A schematic diagram of a typical supercell structure for a (111)
surface is depicted in Fig. 4.2a, where the repeated unit cell is indicated by dashed
lines. For ultrahigh vacuum (UHV) or vapor phase systems, the adsorbate is bound
to one side of the slab, as shown in Fig. 4.2a. The interatomic distances of the